With recent advances in electronics technology, there have been demands for a decrease in the size and an increase in the capacitance of multilayer ceramic capacitors. To meet these demands, the thicknesses of dielectric ceramic layers of multilayer ceramic capacitors are decreased. However, a decrease in the thickness of dielectric ceramic layers results in a relative increase in the intensity of the electric field applied to the dielectric ceramic layers. Thus, dielectric ceramics that form dielectric ceramic layers need to have reliability in the presence of voltage, particularly improved lifetime characteristics in high-temperature loading tests.
For example, Japanese Patent No. 4407497 (Patent Document 1) discloses the following dielectric ceramic composition as a technique interesting for the present invention.
The dielectric ceramic composition contains a main component containing barium titanate, a first auxiliary component containing at least one selected from MgO, CaO, BaO, and SrO, a second auxiliary component containing silicon oxide as a main component, a third auxiliary component containing at least one selected from V2O5, MoO3, and WO3, a fourth auxiliary component containing an oxide of R1 (R1 denotes at least one selected from Sc, Er, Tm, Yb, and Lu), a fifth auxiliary component containing CaZrO3 or CaO+ZrO2, a sixth auxiliary component containing an oxide of R2 (R2 denotes at least one selected from Y, Dy, Ho, Tb, Gd, and Eu), and a seventh auxiliary component containing MnO. The ratio of each of the auxiliary components to 100 moles of the main component is as follows: the first auxiliary component: 0.1 to 3 moles, the second auxiliary component: 2 to 10 moles, the third auxiliary component: 0.01 to 0.5 moles, the fourth auxiliary component: 0.5 to 7 moles (the number of moles of the fourth auxiliary component represents the ratio for R1 alone), the fifth auxiliary component: 0<fifth auxiliary component 5 moles, the sixth auxiliary component: 9 moles or less (the number of moles of the sixth auxiliary component represents the ratio for R2 alone), and the seventh auxiliary component: 0.5 moles or less. The dielectric ceramic composition further contains a plurality of crystal grains, wherein the crystal grains have a Ca diffusion region in which at least the Ca diffuses from the surface to the interior of the crystal grains, and the average depth T of the Ca diffusion region in the crystal grains having an average grain size D50 is controlled in the range of 10% to 30% of the D50.
Although the ceramic composition has improved IR temperature dependency, the dielectric constant-temperature characteristics deteriorate with decreasing thickness of dielectric ceramic layers of a multilayer ceramic capacitor formed of the ceramic composition.
Japanese Unexamined Patent Application Publication No. 2007-131476 (Patent Document 2) discloses a dielectric ceramic composition containing dielectric particles having a main component phase composed mainly of barium titanate and a diffusion phase around the main component phase, wherein variations in the average diffusion depth of the dielectric particles have CV in the range of 5% to 30%, and the average diffusion depth refers to the average depth at which an auxiliary component element in the diffusion phase diffuses from the surface of the diffusion phase toward the center of each of the dielectric particles.
In Patent Document 2, one of the auxiliary component elements is Ca, and the thickness of the diffusion phase in which an auxiliary component element diffuses varies in a predetermined range. Thus, it is argued that the dielectric ceramic composition has balanced characteristics in terms of the relative dielectric constant, high-temperature accelerated lifetime, Tc bias, and IR temperature dependency. However, the reliability of multilayer ceramic capacitors produced from the dielectric ceramic composition decreases with decreasing thickness of dielectric ceramic layers of the multilayer ceramic capacitors.
Patent Document 1: Japanese Patent No. 4407497
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-131476